The invention concerns a station for weighing containers, such as vials and the like, destined for use in a line for filling the containers with liquids or powders.
Machines are known which fill vials with a predetermined liquid or powder product. These machines generally exhibit a filling line along which the empty vials advance step-wise or continuously, passing by a dosing station where a dosed quantity of the predetermined product is introduced into the vials.
In these filling lines, both the weight of the product introduced into the containers, and the overall weight of the filled containers need to be measured very accurately. This operation is performed not only to identify and discard any containers which do not conform to specifications, but also to regulate the amount of content introduced into the containers with greater accuracy, possibly using suitable feedback control devices.
Traditionally, sample weighing systems, commonly known as statistical weighing systems, are used for this purpose, which measure the weight of selected containers, picked up at predefined time intervals from the transport line of the containers. These systems enable weighing operations to be effected without slowing down the transport line, and thus without interfering with plant production capacity.
In particular, statistical weighing systems are known which use two scales, destined respectively to determine the tare and the gross weight. Each container to be weighed is picked up from the line before filling by suitable gripping means and transferred to the first scales, where the container is weighed. Subsequently, the container is repositioned on the transport line and filled, after which it is picked up by further pick-up means and transferred to the second scales, where it is weighed a second time. The difference between the two values is the net weight of the contents introduced into the container.
International patent application WO 2007/003407 illustrates an apparatus for statistical weighing of containers. This apparatus comprises a transport line of the containers, a first transferring organ for picking up and transferring an individual container to weighing organs and a second transferring organ for picking up the weighed container from the weighing organs and transferring it back to the transport line. The transferring and weighing organs are duplicated, since both the tare and the gross weight must be determined; in addition, two scales must be used. More specifically, the transferring organs comprise a series of supports which rotate about reciprocally parallel axes of rotation which are perpendicular to the plane in which the containers travel; these transferring organs bear pincer-like gripping organs which grip the containers.
However the drawback with the described system is that it does not deliver a high level of measurement accuracy, essentially because of propagation of instrumental errors of the two scales when calculating the difference between the measured values. This drawback is even more significant when the weights are small and great accuracy is required, as is the case when packing pharmaceutical products.
Also known is a statistical weighing method, as illustrated in document EP 1 677 484 belonging to the present Applicant.
Prior art statistical weighing systems are however rather complicated to construct and operate and do not always optimally meet the speed and precision requirements of current automatic machines.